Update text on Nephio landing page to be release-neutral

[liamfallon]

This PR makes minor changes to the landing page of the documentation to make the language release neutral.

[liamfallon]

/assign @tliron @CsatariGergely @efiacor

[nephio-prow]

@liamfallon: GitHub didn't allow me to assign the following users: CsatariGergely.

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/assign @tliron @CsatariGergely @efiacor

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[radoslawc]

/lgtm

[efiacor]

/approve
/lgtm

[nephio-prow]

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